I currently am happy running Arch, but I would like to build a source-based distro (Gentoo, Source Mage), or even LFS if only for the great learning experience involved (as a summer project/hobby) . But as these take a very long time to build, I was wondering if it's possible to build it on a partition on my terrabyte drive, and then when I am done, export an installable .iso of the system for my acutal computer(s) ?
That's totally doable. In fact, that was the gentoo old way. I remember running a livecd, chroot to the hard drive and start compiling from Stage 1!!!!! that way of installation took around 2 days to get a basic system hehehe good old days when I had time...
Any linux installation is "moveable" to another hard drive, you'll just have to set the proper configs on the fstab and the boot loader. That's all you need :) (and the support for your hardware on the kernel, obviously)